1. Field of the Invention
The present invention pertains to a ventilator and a method of controlling a ventilator to supply a desired inspiratory target volume of fluid, such as air or an oxygen mixture, to a patient.
2. Description of the Related Art
It is known to utilize a conventional ventilator to deliver a fluid, such as air or an oxygen mixture, to a patient in a volume targeted ventilation mode in which the ventilator attempts to deliver to the patient, during inspiration, a preset volume of fluid. To adjust the volume of fluid delivered to the patient during inspiration to achieve this target volume during each inspiration, the ventilator adjusts the pressure of the fluid supplied to the patient. For example, for a given inspiratory phase in a plurality of respiratory cycles, increasing or decreasing the pressure increases or decreases the volume of fluid delivered to the patient, respectively.
Volume ventilators that operate in a volume targeted ventilation (“VTV”) mode monitor the actual volume of fluid delivered to the patient during an inhalation and increase or decrease, as needed, the pressure at which the fluid is delivered to the patient to meet a target volume of fluid. A problem with adjusting the volume of fluid delivered for one inhalation based upon a difference between the volume of fluid delivered during a previous inhalation and the target volume of fluid is that large differences between the previous volume and the target volume can result in large changes in the volume of fluid delivered to the patient. Such changes can result in the patient experiencing uncomfortable and unnatural variations in the volume of fluid received from one inhalation to another.
It is also known to operate a ventilator in a volume assured pressure support (“VAPS”) mode in which the pressure is controlled by the ventilator in a manner so as to ensure that a set minimum volume is always delivered to the patient during each breath. In this mode of volume ventilation, if, during an inspiratory phase, the patient's inspiratory flow is not sufficient to provide the set volume for that breath, the ventilator transitions to a volume controlled mode of operation and increases the pressure of the fluid flow to the patient to meet this set volume. This typically occurs at the middle or near the end of the inspiratory phase when the ventilator determines that the patient's inspiratory rate will not be sufficient to achieve the set volume for that breath. Because this increase in pressure typically occurs near the end of the breath, when the patient is most likely to want to exhale, this mode of ventilation can be uncomfortable to the spontaneously breathing patient.